constrained motions

CONSTRAINED MOTIONS

The three types of constrained motions in kinematic pairs are given in Fig. 1.6.

1. Completely Constrained Motion

• When the relative motion between two links is limited to a definite direction, then the motion is said to be a completely constrained motion.

• In other words, completely constrained motion results in single degree of motion. • Practical examples: (i) Square bar moving in square hole (Fig.1.7(a)), and

(ii) A shaft with collars at its ends turning in a round hole (Fig. 1.7(b)).

2. Incompletely Constrained Motion

• When the relative motion between two links can take place in more than one direction, then the motion is called an incompletely constrained motion.

• Practical examples: A circular shaft moving in a circular hole (Fig.1.8(a)) and a circular shaft moving in a foot-step bearing (Fig.1.8(b)) have two types of motions: (a) rotary, and (b) reciprocating (sliding). Since both motions of rotation and sliding can take place. simultaneously, the motion is incompletely constrained.

3. Successfully Constrained Motion

• When the relative motion between the links is not completely by itself, but it is achieved by some other means, then the motion is said to be successfully or partially constrained motion.

• Practical examples: (i) The incompletely constrained motion of a circular shaft in a circular hole (shown in Fig.1.8(a)) can be successfully constrained to a pure rotational motion (without sliding motion) by providing a key way in the circular shaft, as shown in Fig. 1.9(a).

(ii) The completely constrained motion of a circular shaft in a foot-step bearing (shown in Fig.1.8(b)) can be made successfully constrained to a rotational motion only (without sliding motion) by using a load, as shown in Fig.1.9(b).

• It may be noted that in actual practice, the degrees of freedom of a mechanism can be suppressed (reduced) by means of applications of constraints on the motion of the links.